Inventory Management Strategies and Costs

Definition and Importance
- Big picture perspectives of inventory as both:
- Asset (appears on the balance sheet)
- Cost (appears on the income statement)
- Inventory influences customer service levels.
- Firms strive to:
- Avoid stockouts
- Not tie up excess working capital

Cycle (Batching) Stock
- Relates to purchasing, manufacturing, and shipping in economical lots.
- Achieves scale economies in:
  - Procurement
  - Production
  - Transportation
Safety Stock
- Acts as a buffer against demand and lead-time uncertainties.
- Involves a blend of both art and science in decision-making.
In-Transit & Work-In-Progress (WIP)
- These goods carry costs while they are either moving or waiting for processing.
- Faster transport reduces time but increases transport costs.
Seasonal Stock
- Used to smooth out supply and demand throughout the season.
- Risks include:
  - Storage costs
  - Obsolescence costs
Anticipatory Stock
- Used as a hedge against potential risks such as strikes, shortages, and price hikes.

Natural Tensions among Functions
- Marketing:
- Favors higher inventory to ensure adequate service levels.
- Manufacturing:
- Prefers longer production runs, resulting in higher inventory and lower unit conversion costs.
- Finance:
- Advocates for lower inventory levels to enhance inventory turnover, return on assets (ROA), return on invested capital (ROIC), and improve cash flow.

Four Inventory Cost Buckets
1. Capital Cost:
- Includes opportunity costs and weighted average cost of capital (WACC).
1. Storage Space:
- Covers handling, rent, and utilities, with distinctions between public and private storage.
1. Service Costs:
- Insurance and taxes associated with holding inventory.
1. Risk Costs:
- Concerns for obsolescence, damage, and shrinkage.
  - Additional costs include:
  - Ordering/Setup Costs
  - Stockout Costs
  - In-Transit Carrying Costs (often overlooked, crucial for global operations).

Determine the item value (variable and directly attributable costs).
Sum the component rates/costs:
- Capital cost
- Space cost
- Service cost
- Risk cost
Divide the total by the item value to establish the percentage of carrying cost, adjusting for holding time.

How much to order?
When to order?
Additional considerations include:
- Where to hold inventory
- Which SKUs (Stock Keeping Units) to prioritize

EOQ Model
- Balances ordering costs against carrying costs to determine the optimal order size (Q).
- Reorder Point (ROP) under certainty:
- ROP = demand during lead time; requires perpetual inventory monitoring.
- Under uncertainty, ROP calculation becomes:
- ROP = demand during lead time + safety stock
- Safety stock accounts for variability in demand and lead time.

Orders are placed at fixed time intervals; quantities vary based on current stock levels.
Best suited for stable demand situations but can be risky with volatile demand patterns.

Pull Systems
- Driven by orders with short forecasting horizons, examples include:
- Just-In-Time (JIT)
- EOQ
Push Systems
- Plan and replenish inventory based on forecasts, examples include:
- Material Requirements Planning (MRP)
- MRP II
- Distribution Requirements Planning (DRP)
- Vendor Managed Inventory (VMI)
System scope distinctions:
- System-wide planners: Use MRP/DRP across multiple nodes.
- Single-facility executors: Apply EOQ/JIT strategies at individual locations.

Just-In-Time (JIT)
- Focuses on maintaining zero or minimal inventories.
- Requirements include:
- Short, reliable lead times
- Frequent small lot deliveries
- High quality standards
- Supplier proximity
- Strong buyer-seller relationships
- JIT reduces downstream inventory while increasing the reliance on reliability and quality.
MRP / MRP II
- Time-phased plans for managing dependent demand items based on:
- Master Production Schedule (MPS)
- Bill of Materials (BOM)
- Inventory status
- Coordinates inbound materials and supports operational as well as financial planning.
Distribution Requirements Planning (DRP)
- Outbound equivalent of MRP, ensuring time-phased SKU/DC product replenishment to achieve desired service levels with minimal inventory upset.
- Often combined with MRP for seamless operations.
Vendor Managed Inventory (VMI)
- Suppliers access real-time pulls and manage customer distribution center (DC) inventory to meet agreed ROP/EOQ targets, which leads to proactive shipment creation.

ABC Analysis
- Utilizes the 80/20 principle focusing control efforts primarily on “A” items based on criteria such as revenue, profit, variability, or criticality.
Quadrant Model
- Involves classifying items by value contribution and supply/service risk to tailor inventory policies accordingly.
Square-Root Rule
- Describes the effect of consolidating inventory across N0 to N1 facilities on the total safety stock, expressed as:
- $SS<em>{future} \backsim SS</em>{current} imes rac{ ext{sqrt}(N1)}{ ext{sqrt}(N0)}$
- Assumes constant service levels, normal demand, stable lead times, and minimal inter-facility transfers.

EOQ (Classic):
- $Q^* = ext{sqrt} rac{2DS}{H}$
- Where:
  - D = annual demand
  - S = order/setup cost per order
  - H = annual holding cost per unit
ROP (Under Certainty):
- $ROP = d imes L$
- Where:
  - d = demand rate
  - L = lead time
ROP (With Safety Stock):
- $ROP = d imes L + SS$
- Where SS is often determined by:
  - Target service level × σ of demand during lead time.
Square-Root Rule:
- Derived from the above formulation regarding facilities.

Always present decisions as trade-offs (e.g., transport vs inventory; order/setup vs carrying; service vs cost).
Tailor methods to match types of demand (independent vs dependent), system scope (site vs network), and control philosophy (push vs pull).
Utilize ABC and Quadrant analysis for effective managerial focus and to optimize inventory policies.
Keep in-transit carrying costs in mind, especially for long lead times and international operations.
Recognize that JIT does not equate to free inventory; it necessitates strong lead-time reliability, quality, and supplier/logistics capabilities.